The present invention claims priority to Japanese Application No. P2000-258139, filed Aug. 23, 2000, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a semiconductor laser used in optical disk systems, optic-magnetic disk memory systems, laser beam printers, and other optical information apparatuses and in optical communications and a method of production thereof, more particularly relates to a semiconductor laser of an oscillation wavelength in the visible light region of the 0.6 to 1.5 xcexcm band and a method of production thereof.
2. Description of the Related Art
The maximum output of a semiconductor laser is limited by the catastrophic optical damage (COD) occurring along with a sharp rise of the temperature of the emission surface of the laser light due to the absorption of the laser light. As a high output semiconductor laser able to suppress the absorption of laser light and prevent COD, there is known a semiconductor laser of a window structure. In this window structure type semiconductor laser, a light guide layer of a bandgap larger than an active layer is provided at the emission surface side to suppress the absorption of the laser light.
When fabricating such a window structure type semiconductor laser, usually two times of growth processes are needed, but there is also a window structure type semiconductor laser that can be fabricated by one time of growth process.
For example, in the example disclosed in the Japanese Unexamined Patent Publication (Kokai) No. 6-232309, a ridge extending along a [01-1] direction is provided in advance in a portion (center portion) along a [01-1] direction of a GaAs (100) substrate (tilt angle: xc2x10.1xc2x0). On the substrate, an active layer comprised of a GaInAs/AlGaAs multi-quantum well layer and other layers of the laser structure are deposited by a single growth process using molecular beam epitaxy. The active layer comprised of the GaInAs/AlGaAs multi-quantum well layer formed on the ridge has a high concentration of In in the GaInAs layer and is thick, so the bandgap of the active layer is small only on the ridge. The active layer at the emission surface sides where the ridge is not provided has a relatively large bandgap compared with the active layer on the ridge, so a semiconductor laser of a window structure is realized.
Below, a method for producing such a window structure type semiconductor laser will be depicted.
FIG. 1 is a perspective view of the structure of a substrate for fabricating the above semiconductor laser.
A ridge 112 (height: 3 xcexcm, width: 5 xcexcm, length: 500 xcexcm) extending along the [01-1] direction is formed at a portion (center portion) along a direction [01-1] on an n-type (100) GaAs substrate 111 (tilt angle: xc2x10.1xc2x0). Because of the ridge 112, the substrate 111 is separated into a ridge area 113 formed with the ridge 112 and two non-ridge areas 114 not formed with the ridge at the two sides of the ridge area 113. The length of each non-ridge area is 20 xcexcm. Note that the ridge 112 is able to be formed by photolithography and etching using an etchant including buffered HF:H2O2:H2O (=10:1:10). In this case, the side surfaces 112a of the ridge 112 are the (311)A plane and the (3-1-1)A plane as reported in Applied Physics Letters, 54, pp. 433 (1989).
FIG. 2 and FIG. 3 are respectively a cross-sectional view and perspective view of a window structure type semiconductor laser fabricated using a substrate shown in FIG. 1. Below, the procedure for fabricating this semiconductor laser will be described.
First, an n-type Al0.3Ga0.7As cladding layer 115 (thickness: 1 xcexcm) is formed on the substrate 111 by molecular beam epitaxy at a substrate temperature of 550xc2x0 C.
Next, a multi-quantum well layer 116 including a GaInAs layer (active layer) and a Al0.1Ga0.9As layer (light guide layer) is formed by molecular beam epitaxy at a substrate temperature of 520xc2x0 C.
Further, a p-type Al0.3Ga0.7As cladding layer 117 (thickness: 1 xcexcm) and a p-type GaAs cap layer 118 (thickness: 500 nm) are successively formed by molecular beam epitaxy at a substrate temperature of 550xc2x0 C.
Here, the multi-quantum well layer 116 has a 5-cycle multi-quantum well layer. One cycle worth of the configuration in a non-ridge area 114 includes Ga0.85In0.15As (thickness: 7 nm) and Al0.1Ga0.9As (thickness: 7 nm).
After the above layers are grown, an electrode 119 is formed. In the ridge area 113, a ridge-shaped light guide having a width of 4 xcexcm and extending along the [01-1] direction is formed as shown in FIG. 2 and FIG. 3 by patterning so that the center of the light guide coincides with the center of the ridge structure on the substrate 111. Then, this is cleaved at the center of the non-ridge areas 114 to produce a semiconductor laser having the (01-1) plane as the end surfaces of its resonator. By this, a window structure type semiconductor laser can be fabricated to have a 500 xcexcm long active layer in the ridge area 113 and a 20 xcexcm long light guide layer of an end surface of a resonator in each non-ridge area 114.
In the above description and as shown in Applied Physics Letters, 56, pp. 1939 (1990), when growing a multi-quantum well layer at a portion where a ridge is formed, the In atoms in a (311) plane and (3-1-1) plane diffuse into the (100) plane.
Therefore, compared with the multi-quantum well layer 116 grown in the non-ridge areas 114, the multi-quantum well layer 116 grown in the ridge area 113 has a high In concentration, a large thickness, and a narrow bandgap.
Therefore, the active layer at the emission surface sides has a relatively larger bandgap than the active layer on the ridge, so a window structure type semiconductor laser can be formed.
In the aforethe example of the related art, however, there is a disadvantage that the technique cannot be applied to an AlGaInP laser.
If a laser including P such as an AlGaInP laser is grown on a ridge stripe in the [01-1] direction as mentioned above, abnormal growth occurs around the ridge.
Because the growth rate declines in the region in the vicinity of the abnormal growth area, the growth rate on the ridge stripe declines and the thickness becomes smaller. If the thickness becomes small, the width of the well of the multi-quantum well layer becomes narrower, so the quantum level rises and the bandgap large becomes larger. On the other hand, the In concentration on the ridge stripe becomes higher regardless of abnormal growth, so the bandgap becomes small. There is therefore the disadvantage that the effects of the growth rate and the In concentration offset each other making it difficult to fabricate a window structure using a ridge stripe.
In addition, since the growth rate on the ridge stripe declines compared with that in a non-ridge area (for example, declines by 40% in the case of a ridge 30 xcexcm in width and 2.7 xcexcm in height), there also arises the disadvantage that a large deviation occurs in the positions of the active layer on a ridge stripe and the light guide layer in the non-ridge areas.
Due to this deviation, there are the problems that the light guide is distorted, the wave surface shifts, and the far field pattern becomes asymmetric.
Further, there is the problem that if an AlGaInP laser is formed on a GaAs (100) substrate that is not tilted, the surface morphology degrades.
An object of the present invention is to provide a window structure type semiconductor laser able to suppress abnormal growth in the vicinity of a ridge, having almost no decline in growth rate on a ridge stripe, and having a good surface morphology and a method of production thereof.
To attain the above object, according to a first aspect of the present invention, there is provided a semiconductor laser having a light guide between resonator end surfaces formed by end surfaces of an active layer, comprising a substrate having a surface tilted to a [0-1-1] direction from a (100) plane and a semiconductor stack formed on the substrate and comprising an active layer having two types of Group III elements including at least indium (In) and Group V elements including phosphorus (P) and a cladding layer of a first conductivity and a cladding layer of a second conductivity provided above and below the active layer, respectively, wherein at least one step-like structure is provided on the substrate and the light guide is provided at an upper step side of the step-like structure so that a portion of the light guide not including the resonator end surfaces is positioned in a vicinity of the step-like structure and so that a distance between the resonator end surfaces of the light guide and the step-like structure become greater than a distance between the portion of the light guide not including the resonator end surfaces and the step-like structure.
Preferably, the step-like structure is formed only in a vicinity of the portion of the light guide not including the resonator end surfaces.
Preferably, the step-like structure comprises a step difference of a groove provided in the substrate, and the light guide is arranged at an upper step side of the step difference of the groove so that the portion of the light guide not including the resonator end surfaces is positioned in a vicinity of the step difference of the groove and so that a distance between the resonator end surfaces of the light guide and the step difference of the groove become greater than a distance between the portion of the light guide not including the resonator end surfaces and the step difference of the groove.
More preferably, the groove is formed only in a vicinity of the portion of the light guide not including the resonator end surfaces.
Alternatively, more preferably, the distance between the resonator end surfaces of the light guide and the step difference of the groove are not more than 50 xcexcm.
Preferably, the step-like structure comprises a ridge provided projecting out on the substrate and having a width at least that of the light guide and the light guide is arranged on the ridge so that the portion of the light guide not including the resonator end surfaces is positioned in a vicinity of a step difference of the ridge and so that distance between the resonator end surfaces of the light guide and the step difference of the ridge become greater than a distance between the portion of the light guide not including the resonator end surfaces and the step difference of the ridge.
More preferably, the substrate is provided with a first ridge having a width at least that of the light guide and extending along a direction in which the light guide extends and second ridges intersecting the first ridge, and the light guide is arranged on the first ridge so that the resonator end surfaces of the light guide are positioned in regions where the first ridge and the second ridges intersect.
Alternatively, more preferably, in the regions where the resonator end surfaces of the light guide are positioned, the ridge is formed wider than the region where the portion of the light guide not including the resonator end surfaces is positioned.
Alternatively, more preferably, the width of the ridge is not more than 100 xcexcm.
Preferably, a direction of the resonator is a [01-1]direction of the substrate.
Alternatively, more preferably, an angle of an inclination to the [0-1-1] direction of the (100) plane of the substrate is 2xc2x0 to 15xc2x0.
Alternatively, more preferably, the substrate comprises GaAs, GaP, or InP.
In the semiconductor laser according to the present invention, a bulk active layer or a multi-quantum well active layer having two types of Group III elements including at least indium (In) and Group V elements including phosphorus (P) is formed on a substrate on which at least one step-like structure is provided in the direction of the light guide in a region other than the resonator end surfaces and the light guide.
Concerning the above active layer, at the upper step side of the step-like structure, the In concentration in the active layer becomes higher the closer to the step-like structure and becomes lower the farther. Namely, the In concentration is high in the vicinity of the step-like structure, that is, the portion of the light guide not including the resonator end surfaces, so the bandgap becomes narrow in the active layer in the portion of the light guide not including the resonator end surfaces. On the other hand, the resonator end surface portions of the light guide are farther away from the step-like structure than the portion of the light guide not including the resonator end surfaces, so the bandgap at the resonator end surfaces is large compared with that in the portion of the light guide not including the resonator end surfaces. In this way, the window structure can be realized.
In addition, the above structure can be formed by one crystal growth process.
Further, because the substrate is tilted, the surface morphology is good, abnormal growth is also suppressed to a large extent, and there is almost no change of the growth rate between the vicinity of the step-like structure and the ends of the resonator. Therefore, there is no step difference of the light guide between the vicinity of the step-like structure and the ends of the resonator, and the problems of the shift of the wave surface, the asymmetry of the far field pattern, and so on do not arise.
Because the above step-like structure is formed only in the vicinity of the portion of the light guide not including the resonator end surfaces, the distance between the resonator end surface portions of the light guide and the step-like structure are larger than the distance between the portion of the light guide not including the resonator end surfaces and the step-like structure.
The step-like structure provided on the substrate can be made a step difference of a groove. For example, by forming the groove only in the vicinity of portion of the light guide not including resonator end surfaces, the distance between the resonator end surface portions of the light guide and the step-like structure are longer than the distance between the portion of the light guide not including resonator end surfaces and the step-like structure.
The step-like structure provided on the substrate can also be made a ridge provided projecting from the substrate.
In this case, the In concentration is high on the ridge. The In concentration becomes higher the narrower the ridge and lower the wider the ridge.
Therefore, for example, by providing first ridge extending along the direction in which the light guide extends and second ridges intersecting the first ridge, setting the second ridges wider than the first ridge, and arranging the light guide so that the resonator end surface portions of the light guide are positioned in regions where the first ridge and the second ridges intersect, that is, by cleaving at the second ridges, the bandgaps at the resonator end surfaces in the regions where the first and the second ridges intersect become larger compared with that on the first ridge, so the window regions are realized.
Further, as the shape of the ridge, by forming the ridge at the portions where the resonator end surface portions of the light guides are positioned wider than the region where the portion of the light guide not including resonator end surfaces is positioned, the bandgaps at the resonator end surfaces on the wide ridge can be made larger compared with an active layer on a relatively narrow ridge, so window structure can be realized.
To attain the above object, according to a second aspect of the present invention, there is provided a method for producing a semiconductor laser having a light guide between resonator end surfaces formed by end surfaces of an active layer, comprising the steps of providing at least one step-like structure on a substrate having a surface tilted to a [0-1-1] direction from a (100) plane and forming on the substrate a semiconductor stack having an active layer including two types of Group III elements including at least indium (In) and Group V elements including phosphorus (P) and a cladding layer of a first conductivity and a cladding layer of a second conductivity arranged above and below the active layer, respectively, wherein the light guide is provided at an upper step side of the step-like structure so that a portion of the light guide not including the resonator end surfaces is positioned in a vicinity of the step-like structure and so that distance between the resonator end surfaces of the light guide and the step-like structure become greater than a distance between the portion of the light guide not including the resonator end surfaces and the step-like structure.
Preferably, a groove is provided on the substrate as the step-like structure and the light guide is arranged at an upper step side of the groove so that the portion of the light guide not including the resonator end surfaces is positioned in a vicinity of the groove and so that a distance between the resonator end surface portions of the light guide and the groove are greater than a distance between the portion of the light guide not including the resonator end surfaces and the groove.
Preferably, the step of providing the step-like structure includes a step of providing a ridge that has a width at least that of the light guide projecting from the substrate and arranging the light guide on the ridge so that the portion of the light guide not including the resonator end surfaces is positioned in a vicinity of a step difference of the ridge and so that a distance between the resonator end surfaces of the light guide and the step difference of the ridge are greater than a distance between the portion of the light guide not including the resonator end surfaces and the step difference of the ridge.
Preferably, the step of providing the step-like structure includes a step of providing on the substrate a first ridge having a width at least that of the light guide and extending in a direction in which the light guide extends and second ridges having a width at least a width of the first ridge and intersecting the first ridge and, the step of forming the semiconductor stack includes a step of forming it on the substrate where the first and second ridges are formed by metal organic chemical vapor deposition (MOCVD) and further comprising a step of cleaving the substrate where the semiconductor stack is formed on the second ridges.
Further, more preferably, the first ridge is formed which extends in a [01-1] direction of the substrate and the second ridges are formed which extend in a [0-1-1] direction of the substrate.
Alternatively, the method further comprises, the step of cleaving the substrate on which the semiconductor stack is formed includes a step of cleaving the second ridges at a position at the light guide side from a center line equally dividing the second ridges in a extending direction of the second ridges.
The above method for producing a semiconductor laser according to the present invention provides at least one step-like structure on a substrate having a surface tilted to a [0-1-1] direction from a (100) plane, then forms a semiconductor stack comprising an active layer including two types of Group III elements including at least indium (In) and Group V elements including phosphorus (P) and a cladding layer of a first conductivity and a cladding layer of a second conductivity provided above and below the active layer, respectively.
Here, the light guide is arranged at the upper step side of the step-like structure so that a portion of the light guide not including resonator end surfaces is positioned in the vicinity of the step-like structure and so that the distance between the resonator end surface portions of the light guide and the step-like structure are greater than the distance between the portion of the light guide not including resonator end surfaces and the step-like structure.
According to the above method for producing a semiconductor laser according to the present invention, the In concentration is formed high in the vicinity of the step-like structure, so the bandgap can be formed narrow in the active layer in the vicinity of the step-like structure. On the other hand, the distance between the resonator end surface portion of the light guide and the step-like structure are greater than the distance between the portion of the light guide not including the resonator end surfaces and the step-like structure, so the bandgap at the resonator end surfaces is large compared with that in the active layer in the vicinity of the step-like structure. In this way, a window structure can be formed.
In addition, the above structure can be formed by one crystal growth process.
Further, because the substrate is tilted, the surface morphology is good, abnormal growth is also suppressed to a large extent, and there is almost no change of the growth rate between the vicinity of the step-like structure and the ends of the resonator. Therefore, there is no step difference of the light guide between the vicinity of the step-like structure and the ends of the resonator, and the problems of the shift of the wave surface, the asymmetry of the far field pattern, and so on do not arise.
In the above method for producing a semiconductor laser according to the present invention, the step-like structure provided on the substrate can be made a ridge projecting from the substrate.
In this case, by providing a first ridge extending along the direction in which the light guide extends and second ridges having a width at least that of the first ridge intersecting the first ridge, forming the semiconductor stack, then cleaving at the second ridges, the bandgap at the resonator end surfaces in the regions where the first and the second ridges intersect becomes larger compared with that on the first ridge, so a window structure can be realized.
Further, by cleaving the second ridges at a position at the light guide side from a center line equally dividing the second ridges in the extending direction of the second ridges, the width of the region forming the window structure at the resonator end surfaces is not constrained by the width of the second ridges and thus can be made shorter.